Bearings

ABSTRACT

A plain bearing and method for making the plain bearing are described, the plain bearing having an overlay alloy layer at a sliding surface of the plain bearing, the plain bearing comprising a layer of a strong backing material, a layer of a first bearing alloy bonded to the strong backing material and a layer of a second bearing material comprising said overlay material bonded to said first bearing alloy layer wherein said second bearing material comprises tin having included in the matrix thereof an organic levelling agent.

FIELD OF THE INVENTION

The present invention relates to plain journal bearings, particularlythough not exclusively, for internal combustion engines and to so-calledoverlay coatings deposited upon the running sliding surface of suchbearings.

BACKGROUND OF THE INVENTION

Overlay coatings on plain journal bearings are well known. Such coatingsare used to improve the running characteristics of plain bearings.Generally, overlay coatings are relatively soft metal alloys having ahardness in the region of about 15 Hv; are frequently based on alloys oflead; and, are deposited on another harder bearing alloy at a thicknessin the range from about 10 to 30 μm. Overlay alloys of the type underconsideration are usually applied by electro-deposition from aqueousplating solutions.

The bearings on which the overlays are deposited are of generallycylindrical or, more commonly, semi-cylindrical form as half-bearingshells which support the crankshaft journals of internal combustionengines, for example. Such bearings generally comprise a layer of astrong backing material such as steel, for example, on which is bonded alayer of a bearing material frequently chosen from alloys of aluminiumor copper. The method of attaching the layer of bearing alloy to thestrong backing may be any that is suitable and may include techniquessuch as pressure welding of sheets of bearing alloy to the backing; thecasting of molten alloy onto the backing; or, the sintering of powdersof alloy to the backing, for example, these methods not beingexhaustive. The overlay alloy coating is deposited on the surface of theharder bearing alloy and endows the finished bearing so formed withproperties which include conformability and the ability to embed dirtparticles and so prevent scoring of a shaft journal by particles ofdebris carried in the lubricating oil. Although overlay alloys in theirbulk form are relatively weak alloys, they have the ability when appliedas a thin layer to another, harder bearing alloy to increase the fatiguestrength of a bearing embodying that harder and intrinsically strongerbearing alloy. This is effected due to the conformability of the overlayalloy by being able to deform slightly to accommodate slightmis-alignments, especially in new engines during the “running in” phase,and so spread the load more evenly across the bearing surface area.

As noted above, many conventional overlay alloys are based on alloys oflead. Lead is a toxic metal which will eventually be phased out of useby governmental legislation throughout the world. In order to make thelead-based overlay layer less prone to corrosion in hot engine oilsabout 10 weight % of tin is frequently added or, alternatively, 7 to 10wt % of indium. Indium, however, is relatively very expensive comparedwith tin and tends to be used for more expensive, higher performancevehicles. However, when tin is used in the overlay alloy and isdeposited upon a harder bearing alloy such as copper-lead, for example,a problem exists in that the tin under engine operating conditions tendsto diffuse out of the overlay into the lead of the underlying bearingalloy, as does indium. This is solved by coating the surface of theunderlying, harder bearing alloy with a thin diffusion barrier of about1–3 μm of a metal such as nickel. However, this is not entirelysatisfactory as diffusion still occurs and the overlay still becomesdepleted in tin due to the formation of non-equilibrium intermetalliccompounds such as Ni₃Sn or Ni₃Sn₂ which are not good bearing materialsin the situation where the shaft journal wears through the overlay tothe underlying interface comprising these intermetallic compounds.

With the ever increasing demands placed on bearings by engines havinghigher specific outputs and operating at higher engine revolutions,there has been a demand for these relatively soft overlay alloys to haveimproved wear resistance whilst at least maintaining existing levels offatigue, cavitation resistance and corrosion resistance. This demand hasresulted in the development of so-called lead-tin-copper overlay alloysan example of which is Pb-10Sn-2Cu.

Thus, it is an object of the present invention to provide an overlaylayer which is not toxic and a further object is to provide an overlaywhich does not form undesirable compounds at an interface with anunderlying, harder bearing material. A yet further object is to providean overlay having improved performance over known lead-based overlayalloys.

According to a first aspect of the present invention there is provided aplain bearing having an overlay material layer at a sliding surface ofthe plain bearing, the plain bearing comprising a layer of a strongbacking material, a layer of a first bearing alloy bonded to the strongbacking material and a layer of a second bearing alloy comprising saidoverlay material bonded to said first bearing alloy layer wherein saidsecond bearing material comprises tin having included in the matrixthereof an organic levelling agent.

The tin overlay layer according to the present invention comprisesessentially pure tin in that there are no metallic alloyingconstituents, other than unavoidable impurities, however, the tin isdeposited from a bath containing additions of one or more organicmaterials which have the effect of so-called “levelling” on theelectro-deposited tin layer.

Organic materials which have been tested in bearings of the presentinvention embodying tin overlays include nonylphenolpolyglycolether andpyrocatechol. The content of the organic material in the plating bathhas an influence on the degree of levelling achieved in the depositedtin layer, the degree of levelling being reflected in the surfaceroughness of the tin layer.

At low levels of organic levelling agent, too low for the full benefitof the present invention to be felt, the surface appearance of thebearing surface is one of a generally crystalline appearance havingpools of smooth material distributed over the surface. At a content oforganic levelling agent where the whole surface is smooth, this is thedesirable minimum content.

It is believed that the organic levelling agent is incorporated in thematrix of the deposited tin layer as polymer chains occluded in thematrix structure such as in the form of an organo-metallic tin compound,for example. The polymer chains appear to impart a preferred orientationto the tin atoms during deposition which has been found to give improvedslip properties. Improved slip properties have been evidenced by lowercoefficients of friction in the tin layer compared with ordinary tindeposits without the levelling additions. The surface of the tin overlayof the bearing of the present invention is very smooth giving a lowerdegree of friction against a co-operating shaft journal which in turngives improved compatibility between bearing surface and shaft journalresulting in lower wear rates.

The organic constituent of the tin overlay produces an increasedhardness in the range from about 20 to 30 Hv. Pure tin with no organiclevelling agent, depending upon its condition, has a hardness of about8–12 Hv. The hardness of the tin overlay can be changed depending uponthe content of the organic levelling agent in the plating bath; thelower the content, the lower the corresponding hardness. The reverse isalso true in that as the content of levelling agent increases, so alsodoes the hardness. However, it is possible to have too high a content oforganic levelling agent such that the hardness is too high and highinternal stresses are produced in the deposit which can lead to crackingof the tin deposit. It is intended that the overlay of the bearing ofthe present invention operates in a similar manner to conventionaloverlays in that the overlay layer is sufficiently soft to permitparticles of dirt circulating in the lubricating oil to become embeddedin the overlay so as to prevent such dirt particles from scoring theshaft journal. Whilst the tin overlay of the present invention is harderthan pure tin by a factor of X2 to X3 it is still sufficiently soft toprovide the required characteristic of dirt embeddability thus, thepreferred hardness range is 20 to 30 Hv.

The bearing of the present invention may preferably have an interlayerbetween the surface of the first bearing material and the tin overlay toact as a diffusion barrier therebetween. The metal layer may be of athickness lying in the range from about 0.1 to about 3 μm with athickness of 1 to 2 μm being preferred, however, the actual thickness isof comparatively little importance in terms of bearing performance. Themetal may be selected from the non-exhaustive group including nickel,cobalt, copper, silver, iron and alloys of these metals, for example. Ithas been found that under engine operating conditions the tin overlayreacts with the nickel interlayer over time to form the stableequilibrium intermetallic compound, Ni₃Sn₄, due to the presence ofeffectively an excess of tin. As noted above, prior art lead-10tinoverlays tended to form the unstable, non-equilibrium Ni₃Sn or Ni₃Sn₂compounds which are poor bearing materials and have inferiorcompatibility with a shaft journal and have been blamed in the past forcausing seizure when the overlay has worn through to the interlayer.Ni₃Sn₄ on the other hand is a very good bearing material and thus, theoverlay of the present invention in addition to having superiorresistance to wear and cavitation erosion is also less prone to seizurewhen the overlay is nearing the end of its life. Thus, this unforeseeneffect of generating a good bearing material at the interface is seen asa significant advantage of the bearing of the present invention.

As with known overlay layers, the thickness of the overlay of thebearing of the present invention may lie in the range from about 10 to30 μm with 13 to 18 μm being preferred.

The deposition conditions for tin overlays according to the presentinvention may be varied to produce a range of microstructures. Forexample, analysis of the tin overlay layer by SEM has revealed nodiscernible grain size; even at magnifications of X5000 and X10000 nograins can be resolved. However, coatings having grain sizes of up to 3μm may be produced. It is preferred, however, that a smaller grain sizeis produced as these provide improved bearing properties.

According to a second aspect of the present invention, there is provideda method for the deposition of an overlay layer onto the surface of aplain bearing, the bearing comprising a strong backing material having alayer of a first bearing material thereon, said overlay being depositedupon the surface of said first bearing material, the method comprisingthe steps of: providing a bearing having a surface on which to depositsaid overlay; immersing said bearing in a plating solution having asupply of tin ions and an organic levelling agent in said solution;making said bearing cathodic with respect to an anode in said solution;and depositing an overlay of tin, apart from unavoidable impurities,said tin overlay also having said organic levelling agent included in amatrix thereof.

It is preferred to deposit the tin overlay of the bearing of the presentinvention by using a so-called “slot jig” wherein the bearing is heldwith its joint faces against a back face of the slot jig with the boreof the bearing facing the slot, the bearing axis and slot beinggenerally parallel to each other. The plating solution, in which thebearing and slot jig are immersed, is also then sparged through the slottowards the bearing bore.

In this way it has been found that relatively high current densities of2 to 3 A/dm² may be employed compared with less than 1 A/dm² where thebearing is merely immersed in the plating solution without spargingthereof. Furthermore, the quality of the deposited tin layer is greatlyimproved compared with that produced without sparging. The use of highcurrent density permitted by the slot jig and sparging technique alsoreduces plating time from more than 40 minutes to less than 20 minutes.

A typical plating solution producing a tin/organic material overlay on abearing according to the present invention may have a composition asfollows:

Sn⁺⁺ 32–38 g/l SnSO₄ 58–68 g/l H₂SO₄ 185–210 g/l Cu <50 mg/l Chloride<20 ppm

Levelling agent additions of nonylphenolpolyglycolether (10–25%) in amethanol carrier (2.5–10%) in the range from 18 to 70 ml/l to thesolution specified above have been tested. At the lower end of the rangeit was found that the degree of levelling and hardness increase wasinsufficient whilst at the upper end of the range it was found thatthere was too much inherent stress in the tin deposit and crackingoccurred. It was found that concentration in the range from 25 to 55ml/l gave useful increases in overlay performance with little oracceptable deterioration of the fundamental requirements of an overlayalloy in terms of conformability and dirt embeddability. The content ofpyrocatecol was 2.5–10% and amphoteres tensid 2.5% maximum.

BRIEF DESCRIPTION OF THE DRAWINGS

It has been found that the leveller content has a substantially directlyproportional effect on hardness of the tin deposit. However, a limit ofleveller content is reached after which the hardness of the tin depositremains constant and then actually begins to fall after furtherincreasing the leveller content. Similarly, the leveller content alsohas a directly proportional effect on surface roughness once the effectof the initial substrate roughness and greatly increased surfaceroughness of the initial leveller-free tin deposit have been overcome.

In order that the present invention may be more fully understood,examples will now be described by way of illustration only withreference to the accompanying figures, of which:

FIG. 1 shows a cross section through a part of a schematic bearingaccording to the present invention showing the constituent layers;

FIG. 2 shows a top view of a schematic arrangement of a plating jighaving a bearing being plated with a tin/organic material according tothe method of the present invention;

FIG. 3 shows a histogram of mean thickness loss of overlay vs mainjournal number in an engine test comparing bearings according to thepresent invention and bearings plated with known Pb/In overlays;

FIG. 4 shows a histogram of weight loss vs main journal number ofoverlays of bearings according to the present invention and known Pb/Inplated bearings in a 3000 hour engine test;

FIG. 5 shows a histogram of volume loss of overlays of bearingsaccording to the present invention and known Pb/In and Pb/Sn/Cu overlaysin a hot oil corrosion test;

FIG. 6 shows a histogram of fatigue strength of bearings according tothe present invention having a tin/organic material overlay and knownPb/In and Pb/Sn/Cu overlays;

FIG. 7 shows a histogram of volume loss of overlays of bearingsaccording to the present invention, Pb/Sn/Cu and Pb/In overlays;

FIG. 8 shows a graph of leveller content vs hardness; and

FIG. 9 which shows a graph of leveller content vs surface roughness ofthe deposit on a substrate.

DETAILED DESCRIPTION

Referring now to FIG. 1 which shows a cross section of a small portionof a generalised bearing 10 according to the present invention. Thebearing comprises: a strong backing material 12 (only a part of thethickness of which is shown); a layer of a first bearing material 14bonded to the backing 12; an interlayer 16; and, an overlay layer 18 oftin which includes an organic levelling agent combined in the matrixthereof. The backing layer 12 may be steel, for example, but may be anyother suitable material such as bronze for example if corrosionconditions in the application dictated such. The first bearing materiallayer 14 may be any that is suitable but will generally be chosen fromcopper-based alloys or aluminium-based alloys. The interlayer 16 ispresent to form a diffusion barrier to stop rapid diffusion of the tinfrom the overlay 18 into the bearing alloy layer 14 in the case ofcopper-based alloys 14 and to improve the adhesion of the overlay to thebearing alloy in the case of aluminium-based alloys 14. The interlayerwill generally be deposited by electro-deposition where the overlay isso deposited and may comprise a layer of nickel or other suitablematerial as described hereinabove. In use, the bearing 10 will besubject to temperatures up to about 160° C. At temperatures of 90° C.and above, the tin from the overlay will react with the interlayermaterial to form the stable intermetallic compound Ni₃Sn₄ in the case ofa nickel interlayer. The rate of formation increases as the temperaturerises. The Ni₃Sn₄ layer grows at the expense of the overlay, however,the Ni₃Sn₄ layer is a good bearing material per se with goodcompatibility with the co-operating shaft journal (not shown) and thus,does not present a possible seizure threat. The thickness of theinterlayer 16 generally lies in the range from 1 to 3 μm and thethickness of the overlay 18 generally in the range from 13 to 18 μm.

FIG. 2 shows a top plan view of a schematic arrangement 20 ofelectro-plating apparatus for depositing an overlay 18 on a bearing 10.The apparatus comprises a jig 22 having two plates 24, 26 spaced eitherside of a slot 28. The bearing 10 is held against the plates 24, 26 onits joint faces 30. The jig 22 is immersed in a bath (not shown) ofplating solution 32 as is a tin anode 34 of generally cylindrical form.The bearing 10 is made cathodic by a suitable electrical connection (notshown). A sparging tube 36 having holes 38 is situated vertically in thebath in a fixed relationship to the slot 28. Plating solution is pumpedthrough the tube 36 so as to emerge in jet form, as indicated by thearrows 40, which are directed towards the bore of the bearing 10 throughthe slot 28. Although not apparent from FIG. 2, the jig 22 is elongateas are the anode 34 and sparging tube 36 and there is generally a stackof a plurality of bearings 10 being plated simultaneously.

In the tests results which follow, the overlay was deposited upon therelevant substrate alloy bearing alloy 14 and interlayer 16 from aplating bath having the following composition:

Sn⁺⁺ 32–38 g/l SnSO₄ 58–68 g/l H₂SO₄ 185–210 g/l Cu <50 mg/l Chloride<20 ppm

Levelling agent additions of nonylphenolpolyglycolether (10–25%) in amethanol carrier (2.5–10%) in the range from 32 to 35 ml/l were added tothe above aqueous solution.

The interlayer 16 material was in all cases nickel.

FIG. 3 indicates the results of a 3000 hour test on a Volvo (trade name)diesel truck engine. Main bearings 1 to 4 inclusive were fitted withbearings according to the present invention as described above whilstmain bearings 5 to 7 inclusive were fitted with bearings of the samematerial and construction but having a conventional overlay of Pb-7In.As may be seen from the histogram of FIG. 3, the mean overlay thicknessloss for bearings of the present invention was less than 10% that of theconventional overlay.

FIG. 4 shows the results of the 3000 hour Volvo engine test of FIG. 3 interms of weight loss. Weight loss of the bearings according to thepresent invention was significantly less than 100 mg each for the fourmain bearings on journals 1 to 4 whereas the weight loss of the bearingson journals 5 to 7 was around 1000 mg each.

FIG. 5 is a histogram showing weight loss of overlays in hot oil (whitemedicinal oil which is chosen for its particularly corrosive nature)after 1000 hours at 120° C., the loss being measured in mm³. The bearingmaterial on which the overlays were deposited has a composition CuSn10which was cast onto steel. The overlays were tin as in the presentinvention, Pb-7In and Pb-10Sn-2Cu. As may be seen from FIG. 5, thevolume loss of overlays on bearings according to the present inventionwas about 60% that of Pb-10Sn-2Cu and much less than 10% that of thePb-7In overlay.

FIG. 6 is a histogram showing the fatigue strength of bearings havingthe overlays specified The bearings according to the present inventionwere tested in two forms: one having a thickness of 18 μm at the upperend of the preferred thickness range; and, the second having a thicknessof 14 μm at the lower end of the preferred thickness range. The overlaythicknesses of the prior art Pb-10Sn-2Cu and Pb-7In overlays was 15–16μm. As may be seen from FIG. 6 the fatigue strength of the bearingsaccording to the present invention was significantly greater than theprior art bearings.

Further tests were carried out where the tin overlay having a thicknessin the range from 13 to 18 μm was deposited on bearing materials 14 ofCu-30Pb-1.5Sn and Cu-10Sn gave fatigue strengths of 90 to 103 MPa.

FIG. 7 is a histogram showing wear test results showing volume loss ofoverlay on bearings according to the present invention compared withconventional overlays as described hereinabove. The test conditionswere: temperature 120°; load 8 kg; speed 500 rev/min; duration 10 mins;and a constant flow of 10 W oil at 600 ml/min. As may be seen from FIG.7 the volume loss of overlays according to the present invention is lessthan 50% of Pb-10Sn-2Cu and less than 40% that of Pb-7In.

Tests were also carried out on the cavitation resistance of overlays onbearings according to the present invention. In these tests, the weightloss of the tin overlay of the inventive bearing was 9 mg whereas theweight loss of a Pb-7In overlay under identical conditions was 37 mg.

FIG. 8 shows the effect of leveller content in the plating bath on thehardness of the tin deposit. It may be seen that the hardness increaseslinearly with increasing content of leveller which was the same as thatin the previously described example.

FIG. 9 shows the effect of leveller content on surface roughness of thetin deposit. At low leveller contents below about 2 ml/l of leveller,the high roughness is a consequence of the substrate surface roughnesswhich was an Ra of o.44 and the roughening effect of the initial,substantially leveller-free tin deposit. Once the effect of the levellerwas such that the surface roughness matched that of the substrate thenincreasing quantities of leveller were directly proportional to thesurface roughness.

Thus, relatively low contents of leveller have a strong effect inhardening and smoothing out surface roughness of the tin overlays of thepresent invention.

Thus, it may be seen that the performance of overlays on bearingsaccording to the present invention is greatly superior to the bestconventional overlays deposited by electro-deposition. Where the overlayis deposited upon a lead-free bearing material 14, the bearing of thepresent invention provides a completely lead-free bearing which complieswith future legislation relating to the elimination of lead fromvehicles.

1. A method for the deposition of an overlay layer onto the surface of aplain bearing, the bearing comprising a strong backing material having alayer of a first bearing material thereon, said overlay being depositedupon the surface of said first bearing material, the method comprisingthe steps of: providing a bearing having a surface on which to depositsaid overlay; immersing said bearing in a plating solution having asupply of tin ions and an organic levelling agent in said solution;making said bearing cathodic with respect to an anode in said solution;and depositing an overlay of tin, apart from unavoidable impurities,said tin overlay also having said organic levelling agent included in amatrix thereof.
 2. A method according to claim 1, wherein the overlay isdeposited in a slot jig apparatus.
 3. A method according to claim 2,wherein the plating solution is sparged through the slot towards thebearing bore.
 4. A method according to claim 2, wherein a platingcurrent density lies in the range from about 2 to 3 A/dm².
 5. A methodaccording to claim 1, wherein a plating current density lies in therange from about 2 to 3 A/dm².
 6. A method for manufacturing a plainbearing comprising a strong backing material having a layer of a firstbearing material thereon on which a interlayer material is provided toact as a diffusion barrier, the method comprising the steps of:immersing said plain bearing in a plating solution having a supply oftin ions and an organic levelling agent in said solution; making saidplain bearing cathodic with respect to an anode in said solution; anddepositing an overlay of tin, apart from unavoidable impurities, on thesurface of said first bearing material, said tin overlay also havingsaid organic levelling agent included in a matrix thereof, characterizedin that the organic levelling agent is one of nonylphenolpolyglycoletherand pyrocatechol.
 7. A method according to claim 6, wherein theinterlayer is selected from the group comprising: nickel, cobalt,copper, silver, iron, and alloys thereof.
 8. A method according to claim7, wherein the plating solution has the following composition: Sn⁺⁺32–38 g/l SnSO₄ 58–68 g/l H2SO₄ 185–210 g/l Cu <50 mg/l Cl- <20 partsper million (ppm), and

an organic levelling agent, being nonylphenolpolyglycolether in amethanol carrier in the range front 25–55 ml/l.
 9. A method according toclaim 6, wherein the plating solution has the following composition:Sn⁺⁺  32–38 g/l SnSO₄  58–68 g/l H2SO₄ 185–210 g/l Cu <50 mg/l Cl- <20parts per million (ppm), and

an organic levelling agent, being nonylphenolpolyglycolether in amethanol carrier in the range from 25–55 ml/l.
 10. A method according toclaim 6, wherein the overlay is deposited in a slot jig apparatus.
 11. Amethod according to claim 6, wherein the plating solution is spargedthrough the slot towards the bearing bore.
 12. A method according toclaim 6, wherein a plating current density lies in the range from 2 to 3A/dm².
 13. A method according to claim 6, wherein the plating solutionis sparged through the slot towards the bearing bore and wherein aplating current density lies in the range from 2 to 3 A/dm².
 14. A plainbearing made by the method of claim 6.